Egypt's presidency says diplomatic efforts to end the country's political turmoil have failed and is warning the Muslim Brotherhood will be held responsible for the consequences.

In a statement, the interim presidency said the period of international efforts that began more than 10 days ago had "ended today".

It said the state held the Muslim Brotherhood of ousted president Mohammed Morsi completely responsible for "the failure of these efforts and the later events and developments that might result from this failure".

Supporters of Mr Morsi, who was toppled by the army on July 3, have been staging protest sit-ins in two areas of Cairo for the last five weeks to demand his reinstatement.

Egypt's interim prime minister Hazem el-Beblawi, speaking in a televised statement, said the government had not retreated from its decision to break up the sit-ins.

The protesters had "broken all the limits of peacefulness", he said, listing crimes including incitement of violence, the use of weapons, blocking roads and detaining citizens.

Any use of weapons against policemen or citizens would "be confronted with utmost force and decisiveness," he said.

"We ask them now again, once again, to quickly leave to their homes and their jobs," he said.

Mr Beblawi added that those whose hands "were not sullied with blood" would not face legal action.

Almost 300 people have been killed in political violence since the overthrow, including 80 shot dead by security forces in a single incident on July 27.

Envoys from the United States, European Union, Qatar and the United Arab Emirates have been trying to defuse the crisis and prevent further bloodshed.

On Tuesday, two senior US senators visiting Cairo forecast bloodshed within weeks unless the authorities released prisoners and began a national dialogue including the Brotherhood. Their warning drew an official rebuke and Egyptian media outrage.

In a first reaction, a spokesman for European Union foreign policy chief Catherine Ashton said the Europeans would "continue to do all we can to try and encourage people to get this inclusive dialogue going, that is so important to see a return to the democratic transition in Egypt."

The Dutch foreign minister was the latest foreign emissary to visit Cairo for talks with his Egyptian counterpart, the prime minister and the president and other officials on Wednesday, but his mission appeared to have come too late.